This invention relates to novel optical fibers formed of aluminum borophosphate glass compositions. Optical fibers currently are undergoing intensive development as the transmission link in optical communication systems. Among the properties required of successful fibers are low optical attenuation, low optical dispersion, large numerical aperture and long service life. Present technology utilized two types of glass for optical fibers, simple silicates and complex silicates.
Simple two-component silicate glasses are made by vapor depositing highly purified raw materials onto mandrels which are subsequently heat treated to give fully densified preforms. Fibers are then pulled from the preforms at high temperatures. Refractive index profiles, either step or graded, are incorporated into the preforms by varying the composition of the gas mixture during vapor deposition. Glass compositions most commonly used are germanosilicate core/silica cladding or silica core/borosilicate cladding. These high silica fibers possess favorable properties, including low attenuation (due both to the high purity of the starting materials and to the deep UV cutoff of silica), satisfactory dispersion characteristics and good solarization resistance. However, the high melting temperatures of silica is a disadvantage of these materials. Temperatures on the order of 2,000.degree. C. are required to pull fibers from the preforms. In addition, this high melting temperature restricts the preform preparation method to relatively low temperature techniques such as vapor deposition, since direct melting at 2,000.degree. C. would lead to unacceptable impurity levels in the glass caused by excessive corrosion of the crucible materials. The careful control of temperature and deposition rates of materials is a second production difficulty of the high silica fibers. The complex nature of the co-deposition process limits the number of components which can be included in the glass composition, and in all practical cases to date, the limit has been three separate oxide components. This constraint affords little flexibility in adjusting the relevant physical properties of the glass, primarily refractive index, thermal expansion coefficient and viscosity-temperature relation. Since the refractive index profile is of greatest importance, the glass composition is generally adjusted to optimize that parameter and this precludes any substantial control of the other physical properties of the glass.
Optical fibers can also be manufactured from complex silicates. The processing involves preparing batch quantities of two glasses of distinct composition by standard glass melting methods, taking care to suppress the level of transition metal impurities. The glass are then remelted in a concentric, platinum double crucible and fibers are drawn directly from the melts through a bottom orifice. The melt from the central crucible gives rise to the core of the fiber while that in the annular crucible provides the cladding. Two variations are possible: if the fiber is cooled quickly, a step index fiber results; whereas if the fiber is maintained at a sufficiently elevated temperature, interdiffusion between the core and the cladding occurs and a graded index fiber is produced. The complex silicate glasses largely avoid the difficulties associated with high silica fibers. Specifically, they can be melted at temperatures low enough (approximately 1,500.degree. C.) so that platinum crucibles can be used without introducing excessive impurity concentrations, and the multi-component nature of the glass composition provides adequate flexibility for independently adjusting the glass properties by altering the relative concentrations of the various components. However, complex silicates also have disadvantages for optical fiber applications. First, such glasses are known to be subject to solarization effects which could limit the operational lifetime of the fibers. Second, the relatively shallow UV cutoff of these glasses has two deleterious effects on their optical properties: it gives a relatively high residual (i.e., non-impurity related) absorption coefficient, and it leads to a relatively high optical dispersion, which is particularly undesirable for communications systems using broad-band emitters such as light emitting diodes for light sources.
It would be desirable to provide optical fibers formed of glass compositions having relatively low melting points so that the composition of the fiber can be controlled relatively easily. In addition, it would be desirable to provide optical fibers having good solarization resistance, low optical dispersion and good transparency to ultraviolet light. Furthermore, it would be desirable to provide optical fibers from glass compositions which have satisfactory index profiles while maintaining low internal mechanical stress.